The present invention relates to an upstream channel overload detection circuit for communication from a mobile station to a base station in a communication system based on a CDMA (Code Division Multiple Access) scheme used in a mobile communication system, and a base station apparatus for the system.
In a CDMA mobile communication system using spread spectrum codes, a service area is divided into a plurality of cells as unit areas, and one base station apparatus is installed in each cell. A mobile terminal existing in a cell of the service area communicates with a base station apparatus in the cell through a radio channel or simultaneously communicates with adjacent base station apparatuses in a plurality of cells. This mobile terminal further communicates from the base station apparatus to another mobile station, telephone set, or the like via another radio channel or communication line.
A mobile communication system like the one shown in FIG. 2 is disclosed in Japanese Patent Laid-Open No. 7-298336. Referring to FIG. 2, the mobile communication system is comprised of a mobile terminal 10 and base station apparatus 20. The mobile terminal 10 includes an antenna 11 for communication with the base station apparatus 20, a transmission circuit section 13 and reception circuit section 14 which are connected to the antenna 11 through a transmission/reception switch 12, a control section 15 connected to the transmission circuit section 13 and reception circuit section 14, and an reception/interference level detection section 16 for measuring the reception level of a perch channel and an interference level (corresponding to noise caused by mutual interference) contained in a reception level.
The control section 15 calculates a carrier to noise ratio (CNR) on the basis of the reception level and interference level detected by the reception/interference level detection section 16.
The reception/interference level detection section 16 is comprised of a correlation detection section 17, level detector 18, and interference detector 19. In the reception/interference level detection section 16, the output signal received by the reception circuit section 14 from the base station apparatus 20 in each cell over a perch channel from the base station apparatus 20 through the antenna 11 and transmission/reception switch 12 is supplied to the correlation detection section 17. The correlation detection section 17 sequentially detects the correlations between the perch channel designated by the control section 15 and level monitor spreading codes, and despreads the output signal from the reception circuit section 14 with the corresponding level monitor spreading code. The power spectrum obtained by this despreading is supplied to the level detector 18 and interference detector 19, which then measure a reception level and interference level, respectively. A CNR is calculated from this measurement result, and an optimal cell is selected.
The base station apparatus 20 includes a plurality of antennas 21 for communication with the mobile terminal 10, a base station amplifier (AMP) 26 connected to the antennas 21, a modulator/demodulator (TRX) 27 for demodulating a reception signal and modulating a transmission signal, a spreading unit 28 connected to the modulator/demodulator 27, a control bus control 30 connected to the modulator/demodulator 27, a monitor control unit 31, a 2 M/1.5 M interface unit 32, a base station control unit 33, and a timing supply unit 34.
Although FIG. 2 shows only the transmission system of the base station apparatus 20, the reception system also has a high-frequency amplifying section, frequency conversion section, demodulation section, de-spreading section, and the like (not shown). The base station apparatus 20 designates system control and a communication channel with respect to the mobile terminal 10 by using a perch channel. In accordance with this designation, the mobile terminal 10 measures a reception level and interference level and selects an optimal cell.
In such a conventional mobile communication system using the CDMA scheme, a single frequency is used for communication between a plurality of mobile terminals and a base station apparatus. To communicate with a maximum number of mobile terminals with a single frequency, the transmission power must be control to a minimum necessary power for each channel. On upstream channels from a plurality of mobile terminals to the base station apparatus, in particular, the distances from the respective mobile terminals to the base station apparatus differ from each other. In addition, the mobile terminals move. For these reasons, propagation losses change every moment. If the base station apparatus receives only a signal from a specific mobile terminal with high intensity, the signal interferes with other channels, resulting in a reduction in channel capacity.
For this reason, on upstream channels, high-speed power control is performed on the transmission power of mobile terminals to make each mobile terminal perform transmission with a minimum necessary power. In this upstream channel power control, the base station apparatus generally determines the upstream channel quality of each mobile terminal and transmits a transmission power control signal to each mobile terminal. In general, the quality of an upstream channel is determined on the basis of the ratio of the reception power of a signal from each mobile terminal which is received by the base station apparatus to the total reception power.
In such control, as the number of mobile terminals in communication increases, the total reception power at the base station apparatus increases. For this reason, sufficient communication quality cannot be obtained unless each mobile terminal performs transmission with a higher transmission power. If the number of terminals in communication further increases and an overload state is about to occur, the total reception power in the base station apparatus further increases. Therefore, each mobile terminal cannot ensure sufficient quality.
In this state, a mobile terminal located at a place where a high propagation loss occurs in propagation to the base station apparatus increases its transmission power to a maximum power with which transmission can be performed. In this case, satisfactory quality cannot be obtained on the upstream channel, and some mobile station may be disconnected. In addition, in a complete overload state, necessary upstream quality cannot be obtained in all mobile terminals, and the base station apparatus instructs each mobile terminal to increase the transmission power. Even if, however, mobile terminals increase their transmission powers to the maximum values, many mobile terminals cannot obtain necessary upstream channel quality and fail to communicate, resulting in abnormal disconnection.
In order to avoid such a state, the following method may be used. The error rate on an upstream channel from each mobile terminal in communication is monitored, and an overload state is determined when necessary channel quality cannot be obtained. At this time, connection of any other mobile terminals is inhibited. In a cellular system using CDMA, channel quality is generally controlled to set the frame error rate to one to several %. For this reason, a correct error rate cannot be calculated unless frame error rate measurement is performed over several hundred frames or more. In addition, since a general frame length is 10 ms to 20 ms, it takes several seconds or more for error rate measurement.
The above conventional method suffers the following problems.
First, it takes several seconds or more for frame error rate measurement, and an overload state may occur during the measurement.
Second, the frame error rate is increased by factors other than an overload state (e.g., a mobile terminal abruptly going underground or entering a tunnel), and hence it is difficult to correctly determine an overload state.
It is an object of the present invention to provide an upstream channel overload detection circuit which can easily detect the overload state of a base station apparatus with a simple arrangement without performing any complicated measurement called frame error rate measurement, and a base station apparatus.
In order to achieve the above object, according to the present invention, there is provided an upstream channel overload detection circuit for a CDMA (Code Division Multiple Access) mobile communication system, comprising reception electric field measurement means for detecting an electric field strength levels of reception signals from a plurality of mobile terminals, first storage means for storing an output from the reception electric field measurement means as a noise level when there is no transmission power of the plurality of mobile terminals, second storage means for storing, as a threshold level, an output from the reception electric field measurement means which corresponds to an allowable number of transmissions from the plurality of mobile terminals, and comparing means for comparing the electric field strength level output from the reception electric field measurement means with a sum value obtained by adding the noise level stored in the first storage means to the threshold level stored in the second storage means.